In a mobile telecommunication system, a number of interfaces are defined interconnecting the nodes of the system. Over the interfaces data is transported such as coded speech information. The codes chosen determines the data quality. A low bit rate code gives lower quality and vice versa. Over some interfaces a low bit rate could be acceptable, while this does not apply for others depending on the interference the interface is subjected to.
The GSM standards have defined how the speech for each Adaptive Multi Rate, AMR, codec is to be transported over the Abis interface connecting the base station controller BSC with the radio base station, RBS, or base transceiver station, BTS. AMR has two codec modes: FullRate, FR, and Halfrate, HR. Eight AMR speech codecs are defined, of which all eight can be used on the full rate mode but only the six lowest on the half rate mode. One Remote Transcoder protocol is defined for full rate mode and one for half rate. For full rate a 16 kbit/s Abis path is used while for half rate a 8 kbit/s path is used.
Although Abis bandwidth can be saved by using the half rate mode, the speech quality is significantly reduced on the radio interface when doing so, especially in high radio interference scenarios. High radio interference is typical in the state of the art networks using Fraction Load Planning, FLP with one to one reuse 1/1. The use of 1/1 is becoming more and more common as traffic is increasing.
The eight AMR codecs used for coding the information sent over the Abis interface have the following bit rates, kilobits per second (kb/s):
Full rate, kb/sHalf rate, kb/s4.754.755.155.155.95.96.76.77.47.47.957.9510.212.2
High radio interference AMR FR will adapt to the current situation and select a lower AMR codec. When using a AMR codec of 7.4 or lower in full rate mode, the Abis will stay on using the 16 kbit/s Remote Transcoder protocol, although in principle 8 kbit/s Abis would be enough. This is done so that Abis Remote Transcoder protocol can be kept when the radio environment is improved and a AMR codec of 7.95 and higher is selected.
With the introduction of the EDGE standard the need for more Abis bandwidth per radio timeslot increases. EDGE normally requires 64 kbit/s per radio timeslot. The introduction of EDGE will for many RBS sites require all additional E1/T1 leased line. This is costly for the operator. Operators are now more and more asking for solutions that will enable them to avoid adding more leased lines.
In U.S. Pat. No. 6,400,954 B1 a technique is used to adjust the transmission on the air interface depending on the load of the access network e.g. the Abis interface.